There is an ever increasing need for gas discharge lamps, such as fluorescent lamps, for both commercial and consumer applications. Gas discharge lamps are usually driven by a mains voltage supply source provided by power utility companies. In order to drive a discharge lamp from the mains voltage supply line, a ballast is employed that functions as an interface between the lamp and the supply line.
One main function of a ballast is to drive the discharge lamp with a signal that has an appropriate voltage and current level. Another important function of a ballast is to perform, what is known as, power factor correction. The voltage and current level necessary to operate the discharge lamp is governed, among other things, by the characteristics of the gas contained inside the lamp. Power factor correction is necessary to insure that the operation of the ballast does not contribute noise signals to the power supply line feeding the ballast. Typically, a power factor correction arrangement controls the supply current provided by the ballast such that it remains in phase with the voltage supply line waveform.
With the advent of HID lamps, the ballasts need to also ensure that the discharge lamp is driven by a low frequency current signal, in the range of 1 kHz or less. Driving an HID lamp at high frequencies is usually difficult due to arc instabilities caused by acoustic resonance. This resonance can lead to lamp failure.
FIG. 1 illustrates a circuit diagram of a typical ballast employed to drive an HID lamp. The operation of ballast 10 is very well understood and is not described in detail herein. The ballast circuit includes an upper signal line and a lower signal line each coupled to a respective terminal of mains power supply line. Ballast 10 comprises an EMI filter 36 followed by a full bridge diode rectifier 12 to rectify the ac voltage signal provided by the mains supply line. The rectified signal is then fed to a preconditioner stage, such as a boost converter 14, which operates to shape the ballast supply current, also referred to as mains current, for power factor correction. The preconditioner is followed by an energy storage capacitor 26, which accumulates a dc bus voltage V.sub.bus, which is typically larger than the peak voltage level provided by the mains power supply line. Boost converter 14 includes an inductor 20 having inductance I.sub.L, along the upper signal line of ballast 10, coupled in series with a diode 24, which in turn is coupled to storage capacitor 26. A transistor switch 22 is coupled across inductor 20 and the lower signal line of the ballast. The duty cycle of switch 22 can be controlled so as to operate the boost converter in different operation modes.
For example, boost converter 14 can operate under, what is known as, a continuous conduction mode operation (CCM). During this mode of operation, the average voltage across capacitor 26 is EQU V.sub.26 =V.sub.in /(1-D(t))
wherein V.sub.in is the voltage signal fed to boost converter 14 and D(t) a variable duty cycle of switch 22. A controller (not shown) varies the duty cycle of switch 22 so that the current I.sub.L has a sinusoidal shape that is in phase with the mains voltage supply waveform. Other control operation modes for boost converter 14 include discontinuous conduction mode operation (DCM) and critical discontinuous conduction mode operation (CDCM), which may be employed based on various design considerations. For a continuous conduction mode operation, the average voltage signal across inductor 20 is substantially zero.
Boost converter 14 is followed by a buck converter 16 that is fed by the dc bus voltage signal formed across capacitor 26. A transistor switch 28 couples capacitor 26 to an inductor 32, which in turn is coupled to a filter capacitor 34. A diode 30 is coupled to switch 28 and to the lower signal line of the ballast. The buck converter creates a dc current which drives the lamp through a commutator stage 18.
Commutator stage 18 includes four transistor switches, which interchangeably operate to switch the current signal provided to lamp 36. The commutator inverts the lamp polarity at a low frequency, typically in the 100 Hz range.
One disadvantage with the ballast circuit described in FIG. 1 is that it suffers from a high component count and poor converter efficiency. There has been some effort to reduce the number of component parts of a ballast for driving HID lamps. One approach is to synchronize the lamp current to the power supply voltage frequency, as described in U.S. Pat. No. 5,917,290, entitled Parallel-Storage Series-Drive Electronic Ballast. The disadvantage of such a ballast circuit is that when the frequency of the power voltage supply signal is low, for example 50 Hz, there is the possibility of a visible light flicker from the lamp.
Thus, there is a need for an efficient and simple ballast circuit having a low component count and a driving current signal that has a controllable frequency to avoid possible light flicker.